Former London mayoral hopeful and Richmond Park MP, Zac Goldsmith, has resigned, reducing the government's majority to just 11 seats. His constituency neighbour Dr Tania Mathias, the Tory MP for Twickenham, has vowed to "back Zac" and continue fighting against expansion.

The Tory cabinet is also, very publicly, bristling at the decision, with Putney MP and Education Secretary Justine Greening slamming the "weak economic case" for expansion. The former London Mayor and current Foreign Secretary Boris Johnson has openly criticised Theresa May for her "undeliverable" promise.

The agreement, drawn up in collaboration with British officials, exposes the inadequacy of the UK’s existing climate ambitions and ensures there will be a renewed pressure on the government to bring into UK law a net-zero emissions target.

The government's own Committee on Climate Change (CCC) notes that Britain, in light of the Paris Agreement, must legislate for emissions reductions of "at least 90 per cent below 1990 levels by 2050". Cutting emissions by 90 per cent would give the UK a "carbon budget" of 70 million tonnes of CO2 equivalent (MtCO2e) by 2050; near the most optimistic limits of what is possible using existing technologies and options.

Aviation is a top ten global polluter and emissions from the heavily subsidised industry are set to balloon by 300 per cent if action is not taken sooner rather than later. The historic ICAO aviation emissions deal agreed in Montreal focused not on reducing emissions from engines or replacing fossil fuels, but on relatively weak energy efficiency and dubious "offsetting targets" that won’t be mandatory for many years to come.

Not accounting for airport expansion, emissions from aviation are expected to hit 51MtCO2e in the UK by 2050. In other words, more than two-thirds of the UK's carbon budget will be spent by the aviation industry. And that is without building a new runway at Heathrow.

And this is worse than it looks. The CCC has recommended that aviation emissions are limited to 2005 levels if the UK is to meet its Paris commitments as affordably as possible. That would translate into an emissions cap of 37.5MtCO2e for air travel.

The government can, and must, be held to account for the clear incompatibility between its legal duty to meet the Paris emissions targets and its plan to expand an industry that is already on course to grossly overspend its carbon budget. Environmental lawyers will be scrutinising, in great detail, the climate legislation brought in when the Paris climate agreement comes into force.

We are currently awaiting the results of the second case brought against the government by ClientEarth for its failure to take action on illegal levels of toxic air pollution. It is EU laws, designed in collaboration with British officials to protect British citizens, that the government is violating. Laws that, taking Theresa May's vague assurances at face value, will at least in the short term be written into the UK statute books.

Ministers know they are exposed on air pollution. When David Cameron was at No. 10, his closest adviser, Camilla Cavendish, was at pains to inform him of that fact in no uncertain terms.

Toxic air pollution in the area around Heathrow is already illegally high. And airport-related traffic is a major cause of the deadly levels of toxic air measured across London and the South East.

Although the Davies Commission, which considered the airport expansion, touched upon the issue of air pollution when recommending a third runway at Heathrow, its report made some ambiguous remarks about the relevance of EU air quality laws. According to the prestigious environmental lawyer, Robert McCraken QC, the report is fundamentally "misdirected".

The government's carefully worded announcement on Tuesday suggests that a new runway at Heathrow could be built but left unused until air quality issues are resolved. But these so-called assurances create more trouble for the government, on at least two counts.